1. Field of the Invention
The invention relates to a transponder as installed primarily in maps or marks for identification of objects of various types, but also of persons or animals, and to a method of making such transponders.
2. Discussion of Related Art
Transponders are well known in the art and generally comprise an electrical circuit having a coil connected to an integrated circuit. The transponder is responsive to a received radio frequency signal and produces a radio frequency signal.
U.S. Pat. No. 5,420,757 describes a transponder according to the species in which the electrical circuit is encapsulated in epoxy resin or thermoplastic material. Such a capsule is made by conventional injection molding. This method has a number of drawbacks. In particular, the circuit is exposed to high temperatures and pressures during the molding, which conditions often lead to damage of the circuit. Conventional injection molding typically is conducted at a temperature of 180-400xc2x0 C. and a pressure of 1,000-2,000 bars. Under such conditions, the baking lacquer of the coils softens at high temperatures, so that the coils may disintegrate. Soldered connections may also soften. Since high stresses may be acting on the components because of the high pressure, particularly when they have no connections to which they can be attached during injection molding, they have to be connected to expensive holders or supports made of a suitable material so that they are not displaced in the cavity during the injection molding process. This could lead to incomplete encapsulation and damage in subsequent processing steps. Moreover, the holders penetrate the capsule and then have to be separated by punching or another similar work step. Making an intact capsule becomes more difficult and expensive.
It is one object of the invention to provide an electrical circuit of a transponder having an enveloping capsule.
It is a further object of the present invention to develop a simple and inexpensive method for manufacturing such a transponder.
These and other objects are achieved by the present invention. In the invention, the circuitry of a transponder is encapsulated within a hot melt material having a relatively low melting point. The hot melt encapsulation provides protection of the electronics of the transponder against damage from harsh environments, including those encountered in further processing (injection molding) of the transponder.
More in particular, the invention relates to a transponder comprising an electrical circuit containing at least one component suitable for interaction with an electromagnetic field encapsulated within a capsule, wherein the capsule comprises a thermoplastic resin having a melting point of from 120xc2x0 C. and 250xc2x0 C., and wherein the electrical circuit is encapsulated by the thermoplastic resin.
The invention also relates to a method for manufacturing a transponder comprising an electrical circuit containing at least one component suitable for interaction with an electromagnetic field encapsulated within a capsule, wherein the capsule comprises a thermoplastic resin having a melting point of from 120xc2x0 C. and 250xc2x0 C., and wherein the electrical circuit is encapsulated by the thermoplastic resin, comprising placing at least one of the electrical circuits in a cavity of a mold, and feeding the thermoplastic resin in molten form into the cavity to encapsulate the at least one electrical circuit and form the capsule, wherein the feeding is conducted at a temperature of from 120xc2x0 C. to 260xc2x0 C. and at a pressure of from 5 to 40 bars.